The proliferation of mobile computing devices and wireless local-area networks, also abbreviated to as WLAN's, has created a growing interest in accurate location-aware systems and services.
Of particular interest is the upgrading of existing WLAN infrastructure originally designed for supporting wireless data traffic with a functionality for also enabling location-aware services. The operation (PHY-layer and MAC-layer) of these WLAN's, such as IEEE 802.11 or Hiperlan, is standardized and has originally been designed for wireless data traffic only. Especially WLAN's based on the IEEE 802.11x standard are proliferating and it is highly desirable to add features providing added value. It is therefore of interest to provide a geolocation functionality which does not contradict with the set of standards.
The Global Positioning System (GPS) and wireless enhanced 911 (E-911) mobile phone services address the issue of location finding. However, these technologies cannot provide accurate indoor geolocation, which has its own independent market and technical challenges.
For the determination of a mobile station's location the acquisition of measurement parameters such as received signal strength (RSS) and/or time delay (TD) from a wireless frame exchange between a mobile station and access points can be used.
In their article, Paramvir Bahl and Venkata Padmanabhan, “RADAR: An In-Building RF-based User Location and Tracking System”, IEEE INFOCOM, Israel, March 2000, pp. 775-784, suggest the use of a set of access points (AP) with different locations within a desired location range for providing RSS parameters to a mobile station and to determine the estimated location with triangulation methods using a channel attenuation model.
The prior art also shows pure software solutions where a set of at least three AP's per desired location range is used to provide RSS data for processing with the signature method.
Although an obvious choice, using a multitude of AP's as responding stations within the same location range to provide distance estimates to mobile stations has several drawbacks with respect to WLAN infrastructure installation, data throughput performance, as well as cost and application possibilities of the additional geolocation functionality. Referring to the terminology used in the IEEE 802.11 WLAN standard, an AP provides wireless access within a dedicated area or cell for data traffic to and from stations that are associated with the AP thus forming a basic service set (BSS). Another function of the AP is to coordinate routing data within a distribution system (DS), which in turn is connected to other AP's serving different BSS's, and to a portal for connecting to conventional wired LAN's. At installation time the physical placement of AP's within a building should be carefully chosen to optimize radio coverage and to minimize potential interference between AP's operating on the same or adjacent channels. There is a conflict between the physical placement of an AP serving one BSS within its coverage range, and the requirement for several stations responding to location sensing (by using additional AP's according to prior art solutions) within the same coverage range. The situation is more acute if the WLAN is constituted of several BSS's forming an extended service set (ESS) to cover a given site or campus. Here, the cell planning and frequency allocation with one AP per cell to form an ESS conflicts even more with the suggested use of at least three AP's at locations optimized for best location sensing results. Furthermore, if AP's within audible range are chosen to operate on the same channel, severe mutual interference will occur which may preclude the introduction of future protocol extensions addressing quality of service (QoS) such as IEEE 802.11e.
It is a disadvantage that prior to communicating with an AP, stations are required to authenticate and associate/reassociate themselves, which requires several frame exchanges between station and AP thus causing substantial protocol overhead. With additional AP's used for assisting location sensing the protocol overhead becomes even greater. It is a further disadvantage of the prior art that the stations support the full protocol stack to communicate with the set of AP's. This demands a full set of hardware and software (WLAN adapter card, host computer), thus precluding applications demanding very inexpensive and power-saving hardware such as wireless tags for locating objects.
From the above it follows that there is still a need in the art for an improved determination of the location of a communication device within a wireless network. Moreover, the determination of the position location of a station or tag with an accuracy of a few meters is desired with cost-effective stations or tags.